Optical analysers have long been used for the measurement of parameters of interest within a measurement volume, such as the amount fraction measurements of a chemical component or components of interest within a test medium or the measurement volume temperature. The measurement parameter is also referred to as the measurand. The principles of these optical measurements are well established and will not be discussed in detail within this specification. These optical measurements may typically involve measuring how much light at a particular wavelength or across a wavelength range is absorbed by the chemical component of interest along a particular optical path-length. This is known as absorption spectroscopy and is well known to a skilled person. Reflective optics and/or refractive optics may be used to shape and steer the light beam(s).
In absorption spectroscopy, a light source and an optical detector are placed near (for example on opposite sides of) a measurement volume. The light beam path from the source to the detector may be along a single pass straight line, or may involve one or more reflective passes, such as a multi-pass optical cell in order to increase the absorption path length and hence enhance absorption sensitivity. The light source used may be a diode laser, a quantum cascade laser (QCL), an inter-band cascade laser (ICL), an external cavity laser diode, an external cavity QCL, an external cavity ICL, a light emitting diode (LED) or an incandescent (black or grey body radiation) source, or any other suitable light source known in the art. Depending on the incident light intensity and wavelength and the required sensitivity and time response, the optical detectors may be photodiodes, photomultiplier tubes (PMT), photoresistors or thermal devices such as pyroelectric, thermopile or bolometer detectors, or any other suitable detectors known in the art.
The following illustration will be given for a typical tunable diode laser spectroscopy (TDLS) arrangement for gaseous measurement, but similar considerations and issues may be envisioned for the other optical measurement methods described above and for measuring constituents of any test medium. This particular illustrative example relates to a cross stack gas absorption analyser, with the light source mounted on one side of the stack and the photodetector mounted on the other side. Typically, this will be designed for measurement of a single component within the gas mixture and hence there will be a single diode laser source and a single wavelength sensitive photo-detector. Both the source and the photo-detector must be sufficiently aligned with each other to be able to produce a suitable magnitude of light passed through the stack and incident on the photo-detector to allow the required absorption measurement to take place. The light source and the photo-detector are normally mounted within suitable housings to protect them from the environment, which housings may also contain some or all of the associated electronics and software required to power, control and perform the required measurement algorithms. These housings will typically be mounted to the stack via tubes (nozzles) connected to flanges or some other suitable means, which act as physical support mechanisms in a fixed position and also provide a sealed connection between the stack gas and the ambient environment. Typically, these flanges or attachment means will contain some physical adjustment means for optical co-alignment between the source and the detector by moving the whole source and photodetector housings to different angular positions to compensate for mechanical inaccuracies in e.g. mounting of the nozzles, the flanges and the source and detector enclosures on the flanges.
Dependent on the application, one component measurement may be sufficient. However, for certain applications, measurement of several independent or dependent components may be required. For example, in a combustion application, it might be required to monitor the oxygen concentration and the carbon monoxide concentration in order to optimise burn efficiency. Potentially, the fuel concentration may also be measured to check for flame out conditions and so a total of three measurements such as O2, CO and hydrocarbons (CH4 or C2H4, etc.) would be required. This could be done with two or more independent analysers mounted at separate positions on the stack, either at the same height at different angular positions on the same diameter (see R. M. Spearrin, W. Ren, J. B. Jeffries, R. K. Hanson, Multi-band infrared CO2 absorption sensor for sensitive temperature and species measurements in high-temperature gases, Appl. Phys. B DOI 10.1007/s00340-014-5772-7) or different spatial locations across stack cross-section (see US 2011/0045422 A1 and U.S. Pat. No. 5,813,767) or in the same angular position on the stack, but at different heights (see US 2006/0044562 A1) or finally a combination of all three arrangements. However, mounting all the analysers at different physical locations has a number of disadvantages:    i) The optical beam paths probed by all of the analysers are not equivalent. This means that any dependent parameters do not exactly correlate in either quantity or time. This is especially significant if active feed-back mechanisms are being used, such as to increase or decrease the supply of fuel or oxygen (air) in combustion applications to maintain combustion efficiency at an optimal level. Any lack of correlation or synchronisation could lead to adjustment errors and potentially oscillatory effects, where the feed-back control is continually trying to adjust and correct even under otherwise stable conditions. In this example, this would lead to excess pollution, excess fuel usage and, in the worst case, loss of control of the combustion process with a potentially hazardous outcome.    ii) The installation of the analysers in different locations has increased installation and maintenance costs. These analysers are often in difficult to access locations high up on the stack walls and/or may require special access. Having separate locations will entail extra access time for installation and maintenance. Separate locations will also require extra time and cost for creating the extra access holes and flange attachments.    iii) The analysers may experience different physical ambient conditions such as due to thermal radiation/conduction from the stack wall and exposure to sunlight/shadow/wind, which could lead to significantly different temperature coefficient effects under some circumstances.    iv) The external optical windows of the analysers have to be protected from the aggressive environment of the stack gas. The hazards of this environment include thermal emission from hot stack gas, chemical attack and physical abrasion from particulates. This protection is typically achieved by using a purge gas such as nitrogen, instrument air or other suitable medium, which sweeps past the optical windows and reduces overheating due to heat flow from hot gas stack. The purge gas over external optical windows also helps to keep them clean and unscratched. Having multiple holes in the process wall or stack increases total purge flow consumption and the cost of gas purging.    v) Multiple nozzles in the process wall or pipe and hence multiple windows in contact with the process reduce the robustness of whole measurement system and reduce the safety of the measurements.
An illustration will now be given of the potential increased maintenance costs for an example application. Having two separate locations (entrance and exit holes for the light source and the photodetector) per optical analyser and purge gas supplies for two external optical features increases the cost and complexity of the installation of purge supply. For instance, a typical purge flow rate for protection of a single analyser's optical windows can reach 1000-2000 L/hour per entrance and exit on the heat exchange box of an ethylene cracker furnace. The presence of two or three optical gas analysers with separate entrances and exits leads to a high cost of purge gas supply and complexity of installation of the purge. Thus, in case of installation of three optical analysers with separate entrance and exit holes for each optical analyser, 6000-12000 L/hour supply of nitrogen or instrument air must be arranged. A typical ethylene cracker plant has 12-16 furnaces; therefore installation of three optical analysers on each furnace will lead to very high installation and maintenance costs for only purge gas.
Known systems have typically either provided multiple sources (e.g. diode lasers) and multiple detectors (e.g. photodiodes) within the same housing, or provided an entrance hole and an exit hole for each separate light source. When multiple components are in the same housing, a failure of any one of them is likely to require removal of the complete analyser for repair or replacement, whereas separate entrance holes into the measurement volume for each source and detector has the problems described above. Although optical fibres could be used to direct different laser beams through a single entrance and exit of a measurement volume such as a combustion chamber, there remains a need for careful alignment of the beams.
It is possible with some known systems to achieve synchronised measurement of multiple gases along the same or an equivalent light path, using an analyser that includes multiple components in the same housing. However, these set-ups are inflexible. If a multi-component analyser is installed, it is generally not possible to modify the set-up to detect a different chemical component or set of chemical components that were not in the original specification for the analyser. For example, a prior art multi-component analyser that is installed to measure CO, H2O and CH4 amount fractions cannot be subsequently modified to additionally measure the amount fraction of NO. To achieve this additional measurement, it would either be necessary to remove the existing multi-component analyser and install a new one capable of making the required measurement set, or install a second independent analyser for measuring NO alongside the existing multi-component analyser. The former option is highly undesirable as it is costly and time consuming to perform the required re-fit, and the latter option suffers from the problems noted earlier in respect of independent analysers.
As mentioned briefly above, one problem with multi-component analysers in a single housing is that failure of one measurement channel in a single multi-component analyser will often require dismounting of the whole analyser for repair and therefore loss of all component information being measured for the duration of the repair. It may be necessary to shut down operations while these repairs are carried out, which is highly undesirable.